1. Summary of the Invention
The present invention relates to a display device, in particular a touch panel display device.
2. Description of the Related Art
In recent years, display devices equipped with touch panels (touch panel display devices) have widely been used (see, for example, Japanese Unexamined Patent Publication No. S61-174587 and Japanese Unexamined Patent Publication No. 2003-66417).
The touch panels are classified into various types, such as resistive, capacitive (see, for example, Japanese Unexamined Patent Publication No. 2003-66417), infrared, ultrasonic and electromagnetic inductive touch panels depending on the operation principles. Among them, it has been known that the capacitive touch panels are suitable for display devices because they are relatively less likely to impair optical properties of the display devices.
In general, a capacitive touch panel includes a position sensing transparent electrode arranged to cover a display panel, a plurality of electrode terminals provided on the periphery of the position sensing transparent electrode and a current sensing circuit for sensing currents flowing through the electrode terminals. When a user touches the touch panel, the position sensing transparent electrode is grounded at the touched part via capacitance of a human body. As a result, a value of resistance between the electrode terminals and the grounded part is varied. Then, the variation of the resistance value is sensed by the current sensing circuit to detect the touched position.
In recent years, the display devices including the touch panel display devices are required to be thin and light. In general, an active matrix substrate is formed to protrude over a counter substrate and external circuits such as driver IC chips are mounted on the protruding portion of the active matrix substrate. Therefore, in view of strength of the assembled device and durability to the mounting of the external circuits, the active matrix substrate is formed to be thick enough to ensure the strength of the display device and the durability of the mounting of the external circuit, while the counter substrate is thinned down as much as possible. As disclosed in Japanese Unexamined Utility Model Publication No. 59-9326, this has been a general way to reduce the thickness and weight of the display devices.
The thin counter substrate has been used in conventional thin and light touch panel display devices along with the above-described technical idea. Further, for the purpose of taking light out with high efficiency, the display device is configured to take out light passed through a liquid crystal layer via a counter electrode and the touch panel is arranged on the side of the display device close to the counter electrode.
The conventional capacitive touch panel display device including the touch panel arranged on the counter substrate side has a problem in that the position sensing cannot be performed with high accuracy.